1. Field of the Invention
The present invention relates to a step motor drive circuit and an analog electronic timepiece.
2. Description of the Prior Art
In a background art, there is utilized an analog electronic timepiece for driving to rotate a time hand for indicating time of an hour hand, a minute hand or the like by a step motor. The analog electronic timepiece includes the step motor for driving to rotate the time hand and a step motor drive circuit for driving to rotate the step motor and is constituted to indicate time by the time hand by driving to rotate the step motor by the step motor drive circuit based on a time signal constituting a reference of time (refer to, for example, JP-B-57-18440 and JP-B-63-18149).
FIG. 10 is a circuit diagram of the step motor drive circuit used for the analog electronic timepiece of the background art.
In FIG. 10, the step motor drive circuit includes a first transistor pair comprising a P-channel field effect transistor (FET) 21 and an N-channel field effect transistor 25, a second transistor pair comprising a P-channel field effect transistor 24 and an N-channel field effect transistor 22, driving means (not illustrated) for driving the respective transistors ON/OFF and N-channel field effect transistors 20, 23 respectively connected in parallel with the transistors 21, 24 for a countermeasure of electrostatic resistance. The transistors 20, 23 are always brought into an OFF state.
The step motor drive circuit is constituted by an integrated circuit (IC), and a connection point of the transistors 21, 22 and a connection point of the transistors 24, 25 constitute output terminals Out1, Out2 of the integrated circuit and connected with drive terminals of a step motor 26 for an electronic timepiece.
The driving means rotate the step motor 26 in a predetermined direction by supplying an alternating signal to the step motor 26 by driving the first transistor pair and the second transistor pair alternately in an ON state in driving to rotate the motor. That is, in driving to rotate the motor, the driving means drive the first transistor pair and the second transistor pair alternately in the ON state by bringing the transistors 21, 25 into the ON state and driving the transistors 24, 22 to the OFF state, successively, and by bringing the transistors 24, 22 into the ON state and driving the transistors 21, 25 to the OFF state. Thereby, the step motor 26 is driven to rotate in the predetermined direction by supplying the alternating signal to the step motor 26.
Further, in braking the motor for forcibly holding the step motor 26 at a predetermined rotational position, the driving means short circuit both terminals of the step motor 26 by driving the transistors 21, 24 to the ON state and driving the transistors 22, 25 to the OFF state.
The transistors 20, 23 are maintained always in the OFF state, thereby, the transistors 21, 22, 24, 25 can be protected against destruction or deterioration by static electricity.
However, the transistors 20, 23 are provided with only a circuit protection function and are constituted by transistors having large sizes to promote such circuit protection function. Therefore, the transistors having only the single circuit protection function occupy a large space and therefore, downsizing is difficult. Further, a space utilization efficiency becomes poor during manufacture of the integrated circuit.
Further, when a motor having a large torque is used as the motor 26, a function of supplying a large drive current is requested for the transistors 21 through 25, and transistors having large sizes are needed in addition to the protecting transistors having the large sizes, thereby rendering downsizing difficult to achieve.
It is an object of the invention to provide a step motor drive circuit that is capable of supplying a large drive current to a step motor and is capable of being downsized.
Further, it is another object of the invention to provide an analog electronic timepiece that is capable of using a motor having a large drive current and is capable of being downsized.